Rh Factor

Rh Factor
Karl Landsteiner, an Austrian—later
American—physician discovered ABO
blood groups in 1900. In 1940, 10 years
after receiving the Nobel Prize, he made
still another famous discovery. This was
a blood group called the Rh factor,
named after rhesus monkeys, in which
it was first found. Approximately 85% of
white individuals in the United States
have the factor (positive) and the other
15% do not (negative). The Rh factor is
encoded by a dominant allele at a single
gene. Rh-positive and Rh-negative
bloods are incompatible; shock and
even death may follow their mixing
when Rh-positive blood is introduced
into an Rh-negative person who has
been sensitized by an earlier transfusion
of Rh-positive blood. Rh incompatibility
accounts for a peculiar and often fatal hemolytic disease of the newborn (erythroblastosis fetalis). If an Rhnegative
mother has an Rh-positive
baby (father is Rh-positive) she can
become immunized by fetal blood during
the birth process. Anti-Rh antibodies
are predominately IgG and can cross
the placenta during a subsequent pregnancy
and agglutinate fetal blood.
Erythroblastosis fetalis normally is not a
problem in cases of ABO incompatibility
between mother and fetus because
antibodies to ABO antigens are primarily
IgM and cannot cross the placenta.

The genetics of the Rh factor are very much
more complicated than it was believed
when the factor was first discovered. Some
authorities think that three genes located
close together on the same chromosome are
involved,whereas others adhere to a system
of one gene with many alleles. In 1968 a
revision of the single gene concept listed 37
alleles necessary to account for the phenotypes
then known.Furthermore, the frequency
of the various alleles varies greatly
between whites, Asians, and blacks.

Erythroblastosis fetalis can now be prevented
by giving an Rh-negative mother anti-Rh antibodies
just after the birth of her first child.
These antibodies remain long enough to
neutralize any Rh-positive fetal blood cells
that may have entered her circulation, thus
preventing her own antibody machinery
from being stimulated to produce the
Rh-positive antibodies. Active, permanent
immunity is blocked.The mother must be
treated after every subsequent pregnancy
(assuming the father is Rh+). If the mother
has already developed an immunity, however,
the baby may be saved by an immediate,
massive transfusion of blood free of
antibodies.